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cligen: A Native API-Inferred Command-Line Interface Generator For Nim

This approach to CLIs was inspired by Andrey Mikhaylenko's Python argh (Click became more popular). The key observation is that proc signatures already encode what you need to generate CLIs - names, types, and default values. Reflection then suffices to generate parser-dispatchers translating seq[string] command input into calls to a wrapped proc. In Nim, adding a CLI can be 1-line of code:

proc fun(foo=1,bar=2.0,baz="x",verb=false,args: seq[string]): int=
  ## An API call doc comment
  result = 1      # Of course, real code would have real work here
import cligen; dispatch fun # Whoa..Just 1 line??

Compile it to fun (e.g., nim c fun.nim) and then run ./fun --help to get a minimal (but not so useless!) help message:

  fun [optional-params] [args: string...]
An API call doc comment
  -h, --help                    print this cligen-erated help
  --help-syntax                 advanced: prepend,plurals,..
  -f=, --foo=    int     1      set foo
  -b=, --bar=    float   2.0    set bar
  --baz=         string  "x"    set baz
  -v, --verb     bool    false  set verb

That's it! No specification language/complex arg parsing API/Nim pragma tags to learn. Installing can be as simple as nimble install cligen or even just git clone <thisRepo> and nim c --path:whereYouClonedTo myProgram.nim. If you aren't sold already, here is more MOTIVATION.

The help column is filled with generic set foo placeholders by default. You can override that using parameter-keyed association-list literals:

dispatch fun, help={"foo": "the beginning", "bar": "the rate"}

The same goes for short versions of the CLI arguments. More on that below.

Other invocations (./fun --foo 2 -b=2.7 --baz:"ho"...) work as expected.

Default help tables work with automated "help to X" tools such as complete -F _longopt in bash, compdef _gnu_generic in Zsh, or the GNU help2man (e.g. help2man -N ./fun|man /dev/stdin).

Nim CLI authors who have read this far can mostly use cligen already. Enter --help, --help-syntax/--helps, or illegal commands to get help messages, syntax, exhibit mappings, etc. Out of the box, cligen supports conversion for most basic Nim types (ints, floats, enums, seqs, sets, HashSets of them, ranges, etc.) but can be extended.

CLusage: Param Match/Help Render, Trailing & Required Args

cligen-erated parsers accept any unambiguous prefix for long options. In other words, long options can be as short as possible. In yet other words, hitting the TAB key to complete is unnecessary if the completion is unique. This is patterned after, e.g. gnuplot, gdb, Vim ex-commands, etc. Long options can also be spelled flexibly, e.g. --dry-run|--dryRun, like Nim's style-insensitive identifiers, but with extra "kebab-case-insensitivity". The exact spelling of the key in help controls the look of printed help.

Layout details like column spacing and help colorization are controlled by a CL user config file. Here are screenshots of example night and day themes.)

Most commands have some trailing variable length sequence of arguments like args in the first example. cligen automatically treats the first non-defaulted seq[T] proc parameter as such an optional sequence. cligen applies the same basic string-to-native type converters/parsers used for option values to such parameters.

If a proc parameter has no explicit default value (via =) then it becomes required. The input syntax is the same as for optional values. So, in

proc fun(myRequired: float, mynums: seq[int], foo=1) = discard
when isMainModule: # Preserve ability to `import api`/call from Nim
  import cligen; dispatch fun

the command-line user must give --myRequired=something somewhere. Non-option arguments must be parsable as int with whitespace stripped, e.g. ./fun --myRequired=2.0 1 2 " -3" -- -4.

More general user semantics for argument validation (required-ness, length, "sub-parsing", etc.) can be done like UserError.nim.

Custom Parameter Types or Parsing

While cligen/argcvt supports basic Nim types, the parsing/printing system is extensible via in-dispatch scope argParse & argHelp overloads for types. A simple example:

import std/times
proc foo(i = 42, d = now()): void = echo d

when isMainModule:
  import cligen/argcvt, cligen
  proc argParse(dst: var DateTime, dfl: DateTime,
                a: var ArgcvtParams): bool =
    try: dst = times.parse(a.val, "yyyyMMdd")
    except TimeParseError:
      echo "bad DateTime: ", a.val; return false
    return true

  proc argHelp(dfl: DateTime, a: var ArgcvtParams): seq[string]=
    @[a.argKeys, "DateTime", getDateStr(dfl)]

  dispatch foo, help={"i": "favorite number", "d": "birthday"}

These two "argument IO" procs take a shared coordination parameter for argument metadata (more fully documented in cligen/argcvt.nim). As just a simple e.g., a.parNm is the name of the parameter being parsed. While not usually useful, this can be used to parse the same Nim type differently (based on the name), detect when CLusers alter parameters from defaults, count uses, etc.

This same mechanism can override existing cligen/argcvt parser/helpers if the built-ins are not what you want. Consulting cligen/argcvt for examples is a good idea. Ambitious CLauthors can re-define all conversions by creating their own private cligen/argcvt.nim in their project directories.

Subcommands, dispatch to object init

dispatchMulti lets you expose two or more procs with subcommands a la git or nimble. Each [] list in dispatchMulti is the argument list for each sub-dispatch. Tune command syntax and help strings in the same way as dispatch as in:

proc foo(myRequired: int, mynums: seq[int], foo=1) = discard
  ## Some API call
proc bar(yippee: int, myFlts: seq[float], verb=false) = discard
  ## Some other API call
when isMainModule:
  import cligen
  dispatchMulti([foo, help={"myRequired": "Need it!"}], [bar])

With the above in cmd.nim, CLI users can run ./cmd foo -m1 or ./cmd bar -y10 1.0 2.0. ./cmd or ./cmd --help print brief help messages while ./cmd help prints a comprehensive message, and ./cmd SUBCMD --help or ./cmd help SUBCMD print a message for just SUBCMD (e.g. foo|bar).

Like long option keys or enum value names, subcommand names can also be any unambiguous prefix and are case-kebab-insensitive. So, ./cmd f-O -m1 would also work above.

Rather than dispatching to a proc and exiting, you can also initialize the fields of an object/tuple from the command-line with initFromCL which has the same keyword parameters as the most salient features of dispatch:

type App* = object
  srcFile*: string
  show*: bool
const dfl* = App(srcFile: "junk") # set defaults!=default for type

proc logic*(a: var App) = echo "app is: ", a

when isMainModule:
  import cligen
  var app = initFromCL(dfl, help = { "srcFile": "yadda yadda" })
  app.logic # Only --help/--version/parse errors cause early exit

Common Overrides, Program Exit, Config File/Environment Vars

You can manually control the short option for any parameter via short:

dispatch fun, short={"bar" : 'r'}

If you'd like to define short or help parameters outside of a dispatch call, you need an explicit conversion to a Table symbol:

import cligen
from std/tables import toTable
  Help  = { "foo": "the beginning", "bar": "the rate" }.toTable()
  Short = { "foo": 'f', "bar": 'r'}.toTable()
proc fun(foo = 1, bar = 2, baz = "hi") = discard
dispatch(fun, help = Help, short = Short)

With that "bar" gets 'r' while "baz" gets 'b' as short options. To suppress a long option getting any short option, specify a short key '\0'. To suppress all short options, give short a key of "".

To suppress API parameters in the CLI, pass suppress = @["apiParam", ...]. (For object init, "ALL AFTER field" stops fieldPairs iteration.) To suppress presence only in help messages use help={"apiParam": "CLIGEN-NOHELP"}. Pass implicitDefault = @["apiParam",...] to let the CLI wrapper default API parameter values with no explicit initialization to the Nim default for a type.

The default exit protocol is (with boolean short-circuiting) quit(int(result)) or (echo $result or discard; quit(0)). If echoResult==true, it's just echo $result; quit(0), while if noAutoEcho==true it's quit(int(result)) or (discard; quit(0)). The ors above are based on whether the wrapped proc has a return type or $ defined on the type. So,

import editdistance, cligen # gen CLI for a Nim stdlib API
dispatch(editDistanceASCII, echoResult=true)

makes a program to print edit distance between two required parameters while without echoResult it would be in the shell $? variable.

If these exit protocols are inadequate then you may need to call dispatchGen() and later call try: dispatchFoo(someSeqString) except: discard yourself. This is all dispatch itself does. Return types and values of the generated dispatcher match the wrapped proc. { Other parameters to generated dispatchers are for internal use in dispatchMulti and probably don't matter to you. } A dispatcher raises 3 exception types: HelpOnly, VersionOnly, ParseError. These are hopefully self-explanatory.

If you want cligen to merge parameters from other sources, like a per-program config file and/or $CMD environment variable, then you can redefine mergeParams() after import cligen but before dispatch/dispatchMulti:

import cligen, os, strutils # multi foo/multi bar are like subcommand example
proc mergeParams(cmdNames: seq[string], cmdLine =
                 commandLineParams()): seq[string] =
  let e = cmdNames.join("_").toUpperAscii.getEnv # $MULTI_(FOO|BAR)
  if e.len>0: e.parseCmdLine & cmdLine else: cmdLine # os.parseCmdLine
dispatchMulti([foo, short={"verb": 'v'}], [bar])

You can include cligen/mergeCfgEnv between import cligen & dispatch to merge ${CMD_CONFIG:-${XDG_CONFIG_HOME:-$HOME/.config}}/CMD (with Nim stdlib's parsecfg module) and then $CMD with parseCmdLine as above.

cligen programs look for ${XDG_CONFIG_HOME:-$HOME/.config}/cligen, e.g. ~/.config/cligen/config which allows command-line end users to tweak colors, layout, syntax, and usage help templates.


  1. Can only wrap basic funcs/procs -- no auto|var types, generics, etc.

  2. Param types used must have argParse, argHelp in scope (including generic parameters like the T in seq[T]).

  3. No param of a wrapped proc can be named "help". (Name collision!)

Even More Controls and Details

cligen grew many features upon request & usage. The test suite has suggestively named examples. There's also a Wiki, generated docs for various modules (which include various system interfaces useful in writing CLIs) and a set of fully worked utilities which may be useful to you in their own right. I also try to mention breaking changes & new features in RELEASE-NOTES.